Definition 5.18 [1]}. A function \(f: D\rightarrow \mathbb {C}\)  defined on an open set \(D\)  in the complex plane is said to be \(holomorphic\)  on \(D\)  if the limit
\(f^{\prime }(z)=\lim _{h\rightarrow 0}\frac{f(z+h)-f(z)}{h}\) 
